Graeme E. Murch

Self-diffusion in austenitic Fe-Cr-Ni alloys

The simultaneous lattice diffusion of 59Fe, 51Cr and 57Ni tracers has been measured in the alloys Fe-15Cr-20Ni, Fe-15Cr-45Ni, Fe-22Cr-45Ni and Fe-15Cr-20Ni-1.4Si between 960 and 1400 degrees C. In all four alloys at all temperatures, DCr>DFe>DNi, with DCr/DNi approximately=2.5 and DFe/DNi approximately=1.8. The activation enthalpies for diffusion of the major constituents in any one alloy are quite close to each other. No correlation is found between swelling under irradiation and the diffusion behaviour of the major constituents. The results are discussed on the basis of the random-alloy model. Grain-boundary diffusion is found in these alloys at surprisingly high temperatures.

The haven ratio in fast ionic conductors

Abstract Measurements of the Haven Ratio are conventionally performed in order to extract unique information about the diffusion mechanism in ionic conductors. In this review article, the historical background for such measurements, their present status, results, and the current theories of interpretation are discussed in detail with emphasis on the special problems found in fast ionic conductors. Likely directions for future work are also pointed out.

Diffusion in nanocrystalline materials

Abstract The problem of calculating the effective diffusivity in nanocrystalline materials is addressed. Two simple two-dimensional grain patterns are constructed and several equations are derived from the Hart–Mortlock and Maxwell–Garnett equations. Monte Carlo computer simulation is used to test the validity of the various equations. It is shown that one of the derived equations provides an excellent description of the effective diffusivities up to a volume fraction of atoms in grain boundaries of 40%.

Diffusion correlation effects in non-stoichiometric solids

Abstract Diffusion in lattice arrays reducible to cubic symmetry can be simulated by Monte Carlo procedures. Tracer correlation factors can be derived for diffusion mechanisms in such arrays which include simple solids with point defects and non-stoichiometric materials with complex defect structures subject to defect-defect interactions. It is shown that such interactions can substantially reduce the correlation factor. A cooperative diffusion mechanism involving Wilis 2:2:2 defect clusters in UO2+x is shown to have a correlation factor greater than unity.

A new hybrid scheme of computer simulation based on Hades and Monte Carlo: Application to ionic conductivity in Y3+ doped CeO2

Abstract The computer method of Lattice Statics (HADES/CASCADE) was used to calculate anion migration energies for 30 unique local environments involving the distribution of Y 3+ ions in CeO 2 . The Monte Carlo method was then used to statistically sample the energies in order to calculate the d.c. ionic conductivity. The general trends of the experimental behavior of the ionic conductivity and associated activation energy were followed.

Diffusion correlation in random alloys

Abstract The solid state diffusion process in random binary alloys has been simulated on a computer array of A and B atoms, coded in the relevant cubic symmetry. The process was a Monte Carlo sequence on which were superimposed the appropriate conditions of concentration and defect-atom exchange frequencies. Correlation factors for atoms and vacancies have been obtained for b.c.c. and f.c.c. random alloys. Their validity was verified for monatomic solids of the same symmetry, for which correlation factors are known to a high degree of accuracy from random walk analysis. The reproducibility of the simulation method varies between 4% and 1% contingent upon the degree of correlation. Semi-empirical polynomials of degree z−1 (z is the coordination number for the particular lattice symmetry) for − t versus concentration describe the results obtained accurately (t is defined by ƒ = (1+t)/(1−t)) . Comparison with the calculations by Manning shows discrepancies which are only significant when there is considerable correlation. There are, however, substantial differences between the present results and those of Kikuchi and Sato over the entire concentration and frequency ranges.

Cellular and porous materials: thermal properties simulation and prediction

Interfacial Heat Transport in Highly Permeable Media - A Finite Volume Approach Effective Thermal Properties of Hollow-Sphere-Structures - A Finite Element Approach Thermal Properties of Composite Materials and Porous Media: Lattice-Based Monte Carlo Approaches Fluid Dynamics in Porous Media - A Boundary Element Approach Analytical Methods for Heat Conduction in Composites and Porous Media Modeling of Composite Heat Transfer in Open-Cellular Porous Materials at High Temperatures Thermal Conduction Through Porous Systems Thermal Property of Lotus-Type Porous Copper and Application to Heat Sinks Thermal Characterization of Open-Celled Metal Foams by Direct Simulation Heat Transfer in Open-Celled Foams Subjected to Oscillating Flow Radiative and Conductive Thermal Properties of Foams On the Application of Optimization Techniques to Heat Transfer in Cellular Materials

Computer simulation of ionic conductivity Application to β″-alumina

Abstract Ionic conductivity in β′-alumina has been simulated using a modified Monte Carlo method. By calculating the average drift distance of the sodium ions in an electric field, it has been possible to gain access to the correlation factor of the charge carriers as a function of Na concentration, temperature and nearest neighbour interaction energy. It is shown that this factor, which was implied by Sato and Kikuchi (1971) to equal unity at all temperatures and concentrations, has a temperature dependent minimum in the vicinity of a sodium concentration of 0·5. The minima in the tracer correlation factor (calculated previously) and the charge correlation factor disappear in their ratio (the Haven ratio), which remains between the geometrically dependent limiting values of unity and 1/3 for all temperatures and concentrations.

Diffusional aspects of non-stoichiometry a model for UCx

Abstract The self-diffusion properties of carbon in are described in terms of an f.c.c. lattice gas model in which the C2 groups attract only when they are nearest neighbours. A modified Monte Carlo method is used to calculate the self diffusion coefficient as a function of x and temperature. The diffusion coefficient shows a maximum at about the composition UC1.2 for the four temperatures studied: 2380, 2500, 2600 and 2650 K. The activation enthalpy for diffusion increases non-uniformly from approximately 54 kcals mole−1 (reference point) at compositions close to UC to about 75 kcals mole−1 for compositions near UC2. The functional form of this variation is in satisfactory agreement with existing experimental data.

A Monte Carlo study of diffusion in a model of an ordered adsorbate

Abstract Monte Carlo results are presented for the chemical diffusivity in a lattice-gas model of an ordered adsorbate. The model employed is the square-planar lattice gas with nearest-neighbour repulsive lateral interactions and next-nearest-neighbour attractive lateral interactions at a temperature less than the critical temperature at 50% coverage. Use is made of the recently derived rigorous form of the Darken equation in order to gain insight into the behaviour of the diffusivity. Within the ordered c(2 × 2) phase, a strong maximum in the diffusivity is observed. This phenomenon is shown unequivocally to be a result of a rapid change in the adsorption isotherm within the ordered phase.